If Kelly sustains his career as a 20-35 MPG guy over the next few years, he won't need to be quicker, just smarter/ more experienced to be better (hopefully, he can grow to be an adequate team defender). As far as playing at the '3', I guess it depends on the match-ups. Obviously, quick 3's will get by him like he's standing still. The key to Kelly will be whether his offensive game can blossom -- an outcome I am not convinced will happen. He definitely doesn't have a good feel for when he can get his shot off (defender distance), and this may come in time, but for now it makes him less comfortable shooting shots when guys are within five feet of him. I think he doesn't realize that he is 7-feet tall and that with a slight fadeaway, most guys will not be able to impact his shot. He has to learn to get his shot off quicker and once this happens, his fakes will cause all sorts of problems for defenders. I do think he loses the ball too easily and I'm not sure his weak/slow hands can be improved. This effects his drives and his rebounding in traffic. He almost NEVER comes up with the ball in a contested rebound unless he is able to tip to an open area and get the ball off the bounce.
I probably root harder for Kelly than any other Celtic because I think if he becomes a serious offensive threat he will be a huge part of this team's future. But he has been feast or famine and his inconsistency is concerning.